Among several types of fuel cells, polymer electrolyte fuel cells (PEFCs) are the best suited for transportation vehicles because of fast startup time, low sensitivity to orientation, and favorable power-to-weight ratio. Though relatively low temperature operation at around 80° C. makes fast startup possible, it also requires the use of scarce, expensive platinum-based catalysts especially for the oxygen reduction reaction (ORR) at the cathode. A need exists, therefore, for non-precious metal catalysts suitable for use in fuel cells, which exhibit a catalytic activity similar to precious metal catalysts.
The potential use of non-precious materials instead of Pt in the PEFC cathode has recently received increased attention due to cost analyses that have demonstrated a pressing need. Transition metal-nitrogen-carbon (M-N—C) type catalysts have been studied for almost 50 years since the discovery of their ORR activity in macrocycles bound with transition metals, and considered as the best non-precious metal catalyst to substitute for platinum in PEFCs.
According to previous reports, the most important element of active site(s) in M-N—C catalysts is the nitrogen. Nitrogen in the carbon can exist as pyridinic type (contributing one p-electron to p band), and pyrrole type (contributing two p-electrons to p band). Pyridinic nitrogen can exist only on the edge of the graphene layer, while pyrrolic nitrogen can exist both on the edge of and within the graphene layer. Experimental and quantum mechanical calculation results strongly show that pyridinic and pyrrolic nitrogen are closely related with the activities of M-N—C catalysts.
The effect of sulfur on the ORR catalytic activity has been rarely studied. However, sulfur has a high potential to enhance the activity of ORR in a manner similar to both pyridinic and pyrrolic nitrogen because sulfur resembles the pyridinic nitrogen in that sulfur also has a lone pair of electrons, and sulfur also resembles the pyrrolic nitrogen in that both contribute two p electrons to the pi band of graphene layer.